Particle flux was measured at a standard reference depth of 150 meters using multiple cylindrical particle interceptor traps deployed on a free-floating array for approximately 60 hours during each cruise. Sediment trap design and collection methods are described in Winn et al. (1991). Passively sinking particulate matter is collected, prescreened (335 µm) to remove zooplankton and micronekton carcasses, then the sample materials are analyzed for C, N, P and mass flux (mg m-2 d-1). Samples were analyzed for particulate carbon (PC), nitrogen (PN), phosphorus (PP), and silica (PSi). Typically six traps are analyzed for PC and PN, three for PP, and another three traps for PSi.
A summary of methodology is listed below. Full details can be found at the HOT Field & Laboratory Protocols page. (http://hahana.soest.hawaii.edu/hot/protocols/protocols.html#) or below in Related Publications section (Karl et al., and HOT Program Sediment Trap Protocols: Chapter 18)
1. Principle
Although most of the particulate matter both on the seafloor and in suspension in seawater is very fine, evidence suggests that most of the material deposited on the benthos arrives via relatively rare, rapidly sinking large particles (McCave, 1975). Therefore, in order to describe adequately the ambient particle field and to understand the rates and mechanisms of biogeochemical cycling in the marine environment, it is imperative to employ sampling methods that enable the investigator to distinguish between the suspended and sinking pools of particulate matter. This universal requirement for a careful and comprehensive analysis of sedimenting particles has resulted in the development, evaluation and calibration of a variety of in situ particle collectors or sediment traps. The results, after nearly a decade of intensive field experiments, have contributed significantly to our general understanding of: (1) the relationship between the rate of primary production and downward flux of particulate organic matter, (2) mesopelagic zone oxygen consumption and nutrient regeneration, (3) biological control of the removal of abiogenic particles from the surface ocean and (4) seasonal and interannual variations in particle flux to the deep-sea. Future sediment trap studies will, most likely, continue to provide novel and useful data on the rates and mechanisms of important biogeochemical processes.
At Station ALOHA, we presently deploy a free-drifting sediment trap array with 12 individual collectors positioned at 150, 300 and 500 m. The deployment period is generally 72 hours. The passively sinking particles are subsequently analyzed for a variety of chemical properties, including: total mass, C, N and P.
2. Precautions
Because particle fluxes in oligotrophic habitats are expected to be low, special attention must be paid to the preparation of individual sediment trap collector tubes so that they are clean and free of dust and other potentially contaminating particles. Traps should be capped immediately after filling and immediately after retrieval. Attention should be paid to airborne and/or shipboard particulate contamination sources. In addition, the time interval between trap retrieval and subsample filtration should be minimized in order to limit the inclusion of extraneous abiotic particles and the post-collection solubilization of particles.
3. Field Operations
Hardware
The HOT program free-floating sediment trap array is patterned after the MULTITRAP system pioneered by Knauer et al. (1979) and used extensively in the decade-long VERTEX program. Twelve individual sediment trap collectors (0.0039 m2) are typically deployed at three depths (150, 300 and 500 m). The traps are affixed to a PVC cross attached to 1/2" Spectra (polyethylene) line. The traps are tracked using XEOS and Argos satellite transmitters, VHF radio, and strobelights. Since HOT-71 cruise (insert date here), both the trap array configuration and the deployment period (formerly 72 hours) were altered to conserve ship time.
Trap solutions
Prior to deployment, each trap is cleaned with 1 M HCl, rinsed thoroughly with deionized water then filled with a high-density solution to prevent advective-diffusive loss of extractants and preservatives during the deployment period and to eliminate flushing of the traps during recovery (Knauer et al., 1979). The trap solution is prepared by adding 50 g of NaCl to each liter of surface seawater. This brine solution is gravity filtered through a 0.2 µm filter cartridge after the addition of 10 ml per liter 100% formalin solution. Individual traps are filled and at least 10 liters of the trap solution is saved for analysis of solution blanks.
Post-recovery processing
Upon recovery, individual traps are capped and transported to the shipboard portable laboratory for analysis. Care is taken not to mix the higher density trap solutions with the overlying seawater. Trap samples are processed from deep to shallow in order to minimize potential contamination. The depth of the interface between the high density solution and overlying seawater is marked on each trap and a second mark is made 5 cm above the interface. The overlying seawater is then aspirated with a plastic tube attached to a vacuum system to the upper mark in order to avoid disturbing the high density solution below. Because some sinking particulate material collects near the interface between the high density solution and the overlying seawater, the overlying seawater is removed only to a depth that is 5 cm above the previously identified interface.
After the overlying seawater has been removed from all the traps at a given depth, the contents of each trap is gently mixed to disrupt large amorphous particles and then passed through an acid rinsed 335 µm NitexR screen to remove contaminating zooplankton and micronekton which entered the traps in a living state and are not truly part of the passive flux. The traps are rinsed with a portion of the <335 µm sample in order to recover all particulate matter, and the 335 µm NitexR screen is examined to determine whether residual material, in addition to the so-called "swimmers", is present. If so, the screens are rinsed again with a portion of the 335 µm filtrate. After all traps from the same depth have been processed, the 335 µm screen is removed and placed into a vial containing 20 ml of formalin-seawater solution, and stored at 4°C for subsequent microscopic examination and organism identification and enumeration.
4. Determination of C, N, P and biogenic-Si Flux
The quantities of particulate C, N, P and biogenic silica in the screened trap solutions are determined using methods described in the chapters for Particulate Carbon and Nitrogen, Particulate Phosphorus, and Particulate Biogenic Silica in the HOT protocols document (see Related Publications section below). Six replicate traps are used for C/N determinations and three additional traps for P. Typically, 1.5 to 2 liters are used for a single C/N or P measurement, and a subsample of 250 mL for Si. An equivalent volume of the time-zero sediment trap solution, filtered through the appropriate filters is used as a C, N or P blank
Addendum - PPO4 protocol (April 7, 2015)
The method used for the analysis of particulate phosphate (PPO4) has been modified and applied to samples analyzed from November 2011 (HOT 236) to the present. The previous protocol was in use over at least the previous 10-year period. The modified procedure included vortexing of the sample prior to a longer leaching time (1 hour versus 30 min) of the GFF filter in 0.15 N HCl at room temperature.
Both the previous and modified procedures were tested in paired analyses on samples collected over one year (12 cruises). The modified procedure resulted in higher yields by approximately 50% for water column samples (integrated 0-100 m: old method 1.00±0.27 mmol P m-2, versus 1.56±0.14 mmol P m-2) and approximately 30% for P-flux estimated from sediment trap samples (old method: 0.31±0.07 mg P m-2 d-1 versus 0.40±0.09 mg P m-2 d-1).
Key to Treatment indicator:
- C = Solutions from individual traps combined and replicate subsamples drawn from this solution;
- I = Individual traps sampled as replicates;
- W = Swimmers picked out before analysis;
- O = Some other treatment.
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